1. Field of the Invention
The present invention relates to an absolute position length measurement type encoder that includes a light-emitting element, a scale for forming bright and dark patterns with pseudorandom codes on projection light of the light-emitting element, and a light-receiving element, and in particular to an absolute position length measurement type encoder the arithmetic operation amount of which is small while being highly accurate.
2. Description of the Related Art
Accurate position control and measurement are indispensable in measuring instruments and apparatuses. Therefore, an absolute position length measurement type encoder capable of executing absolute position measurement over some length has been used. In particular, where high accuracy is required, a photoelectric type encoder has been used.
Generally, an absolute position length measurement type photoelectric encoder has an absolute pattern (hereinafter called an ABS pattern) to roughly measure a moving distance and an incremental pattern (hereinafter called an INC pattern) to measure a moving distance at high resolution power by interpolating the interval of a roughly obtained moving distance on a scale (For example, Japanese Published Unexamined Patent Application No. 2003-234941 (hereinafter called Patent Document 1), Japanese Translation of International Application (Kohyo) No. 2005-515418 (hereinafter called Patent Document 2)). And, for example, the scale is attached to a moving stage side, and the light-emitting element and light-receiving element are attached to the base side that supports the stage.
And, since bright and dark patterns formed of ABS patterns and INC patterns, which are formed on the scale, change on the light-receiving element, which receives the bright and dark patterns, in line with movement of the stage, the moving distance can be measured at high accuracy by processing the change in a signal processing circuit. In detail, rough absolute positions are obtained by the ABS pattern, and a minute position between the absolute positions obtained by the ABS pattern is obtained by the INC pattern. That is, it is necessary that the absolute position itself obtained by the ABS pattern be stabilized with high accuracy.
Also, a plurality of patterns whose cycles differ from each other are used as the ABS pattern, and the absolute positions are obtained by the phase relationship thereof. However, as the measurement distance is lengthened, such a problem arises in which the number of the above-described patterns is increased, and the scale and light-receiving element are large-sized. Accordingly, the absolute position length measuring type encoder itself can be downsized by using a single pattern based on a pseudorandom code as the ABS pattern.
However, where the pseudorandom codes are used for the ABS pattern, the arithmetic operation amount is increased for measurement of the absolute position, it takes a large amount of time for the arithmetic operation, and it is not possible to output a result of measurement of a moving distance at a high speed. Describing in detail, this is because, when processing signals of the light-receiving element, which receives bright and dark patterns, in the form of digital signals, for example, the data of one pixel becomes 4 bits where the signal of the light-receiving element is converted by 4-bit grayscale in order to secure the measurement position accuracy of the absolute position, and if an arithmetic operation for correlation is carried out in regard to the entirety of the circulation period data based on the pseudorandom code directly using the pixel data length, the arithmetic operation amount becomes enormous.
Accordingly, with respect to measurement of a moving distance, position measurement based on the INC pattern is regularly carried out without frequently executing position measurement based on the ABS pattern. However, in this case, although the ABS pattern is provided, correct absolute position is not output if erroneous counting occurs by the INC pattern when moving at a high speed. And, erroneous values of the position measurement are continuously output until the next absolute position measurement is carried out. That is, there remains a problem that the reliability in absolute position measurement is low regardless of being the absolute position length measurement type encoder. Further, such an attempt (Particularly, Patent Document 2, etc.) was made in which an absolute position is obtained by binarizing the pixel data with the accuracy of the absolute position maintained by comparing the outputs between the light-receiving elements. However, finally, the process is complicated to cause the arithmetic operation amount to be increased, wherein a problem similar to the above arises.
Also, although the ABS pattern is required to be formed at an accurate interval based on the pseudorandom codes, the patterns may be made thick or thin depending on the conditions of the formation process. In this case, another problem occurs, by which the accuracy of the absolute position obtained by the ABS pattern may be reduced, and becomes unstable.